1. Field of the Invention
The present invention generally relates to a measuring apparatus for a component contained in an exhalation, and more particularly to a measuring apparatus which measures concentration of alcohol contained in an exhalation of a driver of a vehicle.
2. Description of the Related Art
Japanese Patent Publication No. 51-21698 and U.S. Patent Ser. No. 3,855,573 disclose an apparatus which measures concentration of alcohol contained in an exhalation of a driver of a vehicle so as to enable an operation of the vehicle in accordance with a result of the measurement.
This apparatus comprises a tubular member, a suction pump and an alcohol sensor. The suction pump connected to one end of the tubular member, and the other hand of the tubular member is located near a driver's seat of a vehicle. The alcohol sensor is positioned in the tubular member. A portion of an exhalation of the driver is suctioned into the tubular member by the suction pump so that concentration of alcohol contained in the exhalation of the driver is measured by the alcohol sensor. When the concentration of the alcohol measured by the alcohol sensor reaches a predetermined level, the vehicle is rendered not to be operative.
As for the alcohol sensor provided in the tubular member, a heating type alcohol sensor such as described in Japanese Laid-Open Patent Application No. 63-172953 is used. This alcohol sensor comprises a sensing element made of a metal oxide semiconductor such as SnO.sub.2. The sensing element is usually heated at 350.degree. to 400.degree. C. in use.
The above-mentioned conventional technique has some problems described below
A first problem is related to inaccuracy due to air movement of around the alcohol sensor. As mentioned above, the alcohol sensor is used at a high temperature such as 350.degree. to 400.degree. C. If a fluctuation of an air flow is generated around the alcohol sensor, heat is irregularly released from the alcohol sensor, and thus the alcohol sensor cannot be maintained at a constant temperature.
This is also a problem when other sensors are provided near the alcohol sensor so as to obtain more accurate measurement because provision of other sensors may cause a turbulent flow of air passing through the alcohol sensor. Adversely, other sensors may be affected by heat released from the alcohol sensor, and thus an accuracy of measurement may be decreased.
A second problem is related to a condition of exhalation such as a direction and an amount of exhalation. Such condition is dependent on a seating position and a physique of each driver, and thus a variation in results of measurement may occur. It is difficult for the driver to exhale a predetermined amount of air in a predetermined direction every time.
If a suction capacity of the suction pump is set for a condition in which the driver sits at a maximum distance from the inlet port of the measuring apparatus, a sufficient exhalation may be suctioned. However in such a case, accuracy of the alcohol sensor may be deteriorated because large amount of air passes around the alcohol sensor which condition results in undesired cooling of the alcohol sensor to be used at a high temperature as mentioned above. Additionally, waiting time for the alcohol sensor reaching a predetermined temperature may increase due to the cooling effect of the suctioned air. This causes the driver to wait for a long time.
A third problem is related to a possible measuring error due to a mixture ratio of an exhalation of a driver and air in the vehicle. If much air relative to the exhalation is suctioned into the measuring apparatus, concentration of the alcohol to be measured becomes less than a correct concentration of alcohol.
In order to eliminate the above problem, Japanese Patent Publication No. 56-18898 suggested use of a pressure sensor so as to measure a pressure change in the tubular member. This method is effective for preventing a deceiving action such as closing of the tubular member. However, this method is not accurate sufficient for determining whether the exhalation is actually introduced into the tubular member or only surrounding air is introduced by other means.
Japanese Patent Publication No. 56-18898 teaches determining means for determining whether an exhalation is actually introduced in accordance with changes in humidity and temperature of the air to be measured. This method still has a problem in that the result of measurement may be fabricated by putting wet hand over an inlet port for the air to be measured. Additionally, providing sensors other than alcohol sensor in the tubular member may generate a turbulence in the suctioned air which condition affects the accuracy of the alcohol sensor. Further, heating of the alcohol sensor may affect other sensors which may sensitive to a temperature change.
A fourth problem is related to a time spent on a measurement performed by the alcohol sensor. The conventional alcohol sensor takes a relatively long response time (from a time when alcohol makes contact with the alcohol sensor to a time when an output of the alcohol sensor becomes stable at a value corresponding to the concentration of the alcohol). The driver may feel uncomfortable for waiting for such a long response time.